Active sort of ruminant alongside river (4)
I believe the answer is:
doer
'active sort' is the definition.
(I've seen this in another clue)
'ruminant alongside river' is the wordplay.
'ruminant' becomes 'doe' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'alongside' is a charade indicator (letters next to each other).
'river' becomes 'r' (abbreviation).
'doe'+'r'='DOER'
'of' is the link.
Can you help me to learn more?
(Other definitions for doer that I've seen before include "energetic person?" , "Achiever" , "Active type" , "agent" , "Person who acts" .)